Telephone system



Dec. 14,- 1937. H. HOVLAND TELEPHONE SYSTEM 2 Sheets-Sheet 1 Filed June 25, 1 936 /N 1/5 N TOR I H. HO VLAND BV ATTORNEY Dec. 14, 1937. HVLAND 2,102,155

TELEPHONE SYSTEM Filed June 25, 1956 2 Sheets-Sheet 2 INVENTOR H. HOVLA/VD 51 mm i 1 M03 M NH? u a, www

A 7' IORNE V Patented Dec. 14, 1937 E TTEEi ATENT OFFICE Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application June 25, 1936, Serial No. 87,194

'7 Claims.

This invention relates totelephone systems and particularly to those employing automatic switches for establishing connections.

The object of the invention is to improve the 5 efliciency of telephone equipments by reducing unguarded intervals and by minimizing the liability of double connections.

Where trunk circuits interconnecting two offices in a dial system are utilized for traffic in.

both directions, it is necessary to provide some form of busying or guarding means at each end so that when the trunk is seized at one end it will be become busy to the switches or other seizing means at the opposite end. Between the 5 time the trunk is seized at one end and the time it is guarded at the opposite end it is liable to double seizure. Various ways have been proposed heretofore to reducethis unguarded interval.

, According to a feature of the present invention, the unguarded interval of a two-way trunk is greatly reduced by an arrangement in which a circuit is closed directly from the switch that seizes the outgoing end of the trunk over a conductor of the trunk to the distant end, which cirheld in a local circuit in order that it may be disconnected from the trunk to free the latter for the transmission of the impulses that follow immediately. 7

Another feature is a two-way'trunk equipped at each end with a differential guard relay and having'a guard circuit including both of said relays and one of the talking conductors of the trunk which is closed by the switch seizing the 40 trunk at either end, the current flow through said relaysbeing such that only the relay at the distant end of the trunk operates and in operating immediately renders the distant end of the trunk busy.

The foregoing and other features of the invention will be more fully described in the following detailed specification and will also be set forth in the accompanying claims.

Fig. 1 of the accompanying drawings show one end of a two-way trunk circuit arranged according to this invention and associated with conventional circuit representations of a calling subscribers line and the usual line finder and selectors.

Fig. 2 shows the other end of the aforesaid two- Current now flows from said ground through the relay operates and causes the guard relay to be way trunk circuit identical with the trunk of Fig. 1 and associated with conventional circuit representations of a called subscribers line and the usual selectors and connector.

The inventionwill now be described in detail. Let it be assumed that the subscriber of station I00 desires toestablish a telephonic connection with the station 200. Stations I00 and 200 with the associated dials IOI and 20I, respectively, are not shown in detail but are represented conven- 10 tionally. When the receiver at station I00 is removed from its switch-hook, a line finder LF establishes connection with the terminals of line I02 of station I00. When the first two digits of the directory number of the subscriber of station' 200 have been dialed with the dial IN, the connection is thereby extended through step-bystep selectors S1 and S2 to the terminals I03 of the two-way trunk circuit of Fig. 1. Line finder LF and selectors S1 and S2 are of the well-known type found in step-by-step dial offices and are represented in an abbreviated conventional form. When'selector'Sz seizes the terminals I03, ground is connected to the sleeve conductor I04 by means of a relay in the selector, such as relay I30.

lower back contacts and thedifferentially connected series windings of relay I05 through a rectifier I07 to battery I08. The windings of relay I05 are shunted by the resistance I05. The rectifier I01 is preferably of the well-known copper-oxide type, but any other rectifier of suitable characteristics may be used. It passes current in the direction of the arrowhead and blocks current in the opposite direction. Current also now flows from the positive ground of battery 208 of Fig. 2, through the intervening earth between the offices of Fig. 1 and Fig. 2 to the aforesaid ground on sleeve conductor I04. From here the current continues through the lower back contacts and winding of relay I05, upper back contacts of relay I I4, through compensating resistance II 0, lower back contacts of relay III, over the interofiice ring conductor 223, through lower back contacts of relay 2| I, compensating resistance 2I0, upper back contacts of relay 2M, upper and lower windings of relay 205 in multiple, through resistance 206 and rectifier 201 to the battery 208., Since current flows through the lower winding of relay I05 in one direction with respect to the upper winding and in the opposite direction through the lower winding'of relay 205, relay I05 remains inert, and relay 205 attracts its armatures. Due to the current from battery 208 flowing through the lower winding of relay I05, 55

which current is added to that from battery I08, it is necessary to compensate for varying trunk conductor resistances between the two oiiices by so selecting the value of compensating resistance as to obtain suitable non-operating margins for relay i735. It should also be pointed out here that the use of the rectifiers l0! and 201 as shown prevents a possible flow of current over the ring conductor between the two offices, when the trunk circuit not in use, due to a difference of battery voltage or to earth potential efiects. Relay in operating connects ground with its lower inner front contacts to sleeve conductor 204, thereby causing, in accordance with a particular feature of this invention, the terminals 203 to test busy immediately to selectors hunting for an idle outgoing trunk. It is designed to operate quickly so as to reduce the unguarded interval. Relay 2% also connects with its upper and lower outer make contacts the tip and ring conductors 222 and 223 to conductors 2H] and 220, which are connected to the corresponding tip and ring brushes 226 or the selector S3. This prepares an initial circuit for relay 22| traceable from battery through the right-hand winding of said relay 22E, conductor 220, low-er contact of relay 205, conductor 223, thence as above traced through the lower winding of relay I05 to grounded conductor iii l. Relay 22! operates inv this circuit and operates relay 225, which in turn operates relay 216. The purpose of operating relay 2I4 is to provide a circuit for holding the relay 205 locally since its original operating circuit over the trunk will soon be opened by relay [H to prepare for pulsing. The holding circuit for relay 205 may be traced from ground on the armature of relay 224, resistance 225, upper contact of relay 2i i, upper front contact of relay 2H4, windings of relay 205, rectifier 201 to battery. Thus relay 2% at its upper armature severs the original operating circuit and at the same time closes a local holding circuit for relay 205. By this arrangement the seizure of the trunk by selector S2 results immediately in the operation of relay 2% to guard the trunk at the distant end and in the operation of impulse relay HI and relay 2 M to hold the guard relay 205 locally so the trunk can be free from the relay 205 when it is prepared a moment later for pulsing. Should for any reason the relay H I operate before relay 2M operates, the relay 205 is held in a circuit traceable from ground through the left winding of relay 22E, conductor 2l9, upper contact of relay 2G5, thence over the trunk conductor 222, through the bridge at the originating office, back over conductor 223, normal contacts of relay 2H, resistance 228, contact of relay 2M, windings of relay 205 to battery.

Immediately after selector S2 of Fig. 1 seizes the terminals E03 as above explained, relay H2 operates over the circuit established by line finder LF and selectors S1 and S2 through the circuit of station E00 and dial l0l. Relay H2 in operating, closes through, with its upper contacts, the trunk tip conductor and operates relays HI and H3 in multiple through a circuit that is traceable from ground on the lower operated front contacts of relay H2, through the winding of relay M3 to battery and the upper inner nor mally closed break contacts and winding of relay Hi to battery. Relay H3 is designed to operate quickly so as to connect ground with its upper contacts to conductor H6, enabling relay III to lock up through its upper front contacts before its operating circuit before traced is opened by the break of its upper continuity contacts. Ground is also connected by the lower outer contacts of relay l 43 to the sleeve terminal of selector S2, thereby holding operated the preceding switches and guarding the terminals 103 against seizure by another selector. Relay H3 with its upper contacts also closes a circuit from ground through the upper winding of relay l i! to battery. The current flow through this winding, alone, however, is insuflicient to operate relay H1. Through the lower winding of relay H7 another circuit is also closed at this time over the trunk line conductors between the ofiices of Figs, 1 and 2. This circuit is traced from battery through the right-hand winding of relay 22I, over conductor 220, through the lower operated outer contacts of relay 205, over the ring trunk conductor 223, lower operated make contacts of relay I l I, lower back contacts of relay I I4, lower winding of relay Hi, upper operated contacts of relay 1 l2, over the tip line conductor 222, through upper operated contacts of relay 205, over conductor 2E9 and through the left-hand winding of relay 22! to ground. Relay H1, however, does not operate at through its upper and lower windings are magnetically in opposition to each other. Relay 22! is now held in the loop circuit just traced, and its initial operating circuit is opened at the contacts of relay Hi.

The subscriber of station illil will now dial the third digit of the called number. 011 the first break of the contacts of dia'l till, relay H2 releases and opens with its upper make contacts the circuit hereinbefore traced through the lower :l.

winding of relay H? and the windings of the pulse relay 22l of the selector S3 of Fig. 2. Relay l l 2 in releasing also closes a circuit for operating relay H4, which is traced from ground on the lower back contacts of relay H2, lower inner operated contacts of relay H3 and winding of relay [i i to battery. Relay H4 in operating connects with its lower front contacts conductor H5 to tip conductor H8, thereby removing the lower winding of relay ii? from the pulsing circuit. Relays l l3 and i It are slow to release and during pulsing remain operated. Relay 22E follows the pulses of relay H2 and in well-known manner causes selector S3 to establish connection with the terminals of selector Si. is dialed, relay H4 releases. When selector S4 is seized by the brushes 226, relays 224 and 224 are released in well-known manner by the functioning of apparatus in selector S4, and relay 2M is now held by sleeve ground from selector S4. When the fourth, fifth and sixth digits are dialed, the apparatus of Fig, 1 functions in a similar manner to cause selector S4 and connector C to extend the connection to the line of station 260.

When the subscriber of station 200 removes the receiver thereat, a relay in connector C operates and, in accordance with usual practice, reverses the direction of current flow over the tip and ring conductors. Since relay l it releases after dialing the last digit of the called number, the circuit through the lower winding of relay H1 is reestablished, and relay H! now operates as the current through each winding is now magnetically aiding. Relay H? in operating reverses the relation of the windings of relay l I2 with respect to the line conductors extending through to the station 00 in order to establish in wellknown manner a charge condition against station I00 or to give supervision to an operator when this is required. Relay H1 in operating shortthis time since the currents After the third digit lower back contacts of relay It, thereby assuring a more reliable conversational circuit path than would be obtained through the lower back contacts of relay H4 alone. The connection between stations Illl] and'Zllll is now completed. a

When the subscriber of station 1% hangs up the receiver thereat, when the conversation has been completed, .relayl l2 releases, releasing in turn relays H3 andIH, The release of relay H3 removes ground from conductor it, there- 7 by causing the release of the preceding switches, and opens the circuit through the upper winding cussed for a call originating in the oflice of Fig. i

2 and terminating in the ofiice of Fig. 1. Let it be assumed that a calling subscriber, such as the subscriber of station 200, desires to call the subscriber of station I00. The line of station 2% is connected to a line finder and associated selectors, as shown in the case of the line of station me, but not shown for the line of station 280. When the terminals 203 are seized by the brushes of.

selectorss, ground from the'selector is connected to sleeve conductor 204 and current flows from this ground, through the lower inner back contacts of relay Z05, lower and upper windings of relay 205 in series, in multiple with resistance 206, through rectifier. 291 to battery 2%. Current also now flows from the positive ground of battery H18 through the interveningearth between the oihces of Fig. 1 and Fig. 2 to the ground on conductor 2%, thence through the lower inner back contacts and lower winding of relay 205, upper back contacts of relay 2M, compensating resistance 2m, lower back contacts of relay 2! I, thence over the ring conductor to the office of Fig. 1 and through the lower back contacts of relay IH, resistance H0, upper back contacts of relay H4, the upper and lower windings of relay Hi5, through resistance I06, and rectifier Ill! to battery I08. As hereinbefore described in the case ofthe call from station N38 to station 200, relay 205 corresponding to relay N5 of the previous description does not operate due to the differential connection of its windings. Relay H35, however, operates due to the magnetically aiding action of the current in its windings, immediately connects a guarding ground to the sleeve terminal of selector S2, and connects the tip and ring conductors to conductors H9 and I20, respectively. Connection is now established with the line of station I06 through incoming selectors, including selector Se, and a connector, not shown, in the same manner as connection was established with the line of station 200.

The foregoing description is for a completed call. If, in the case of a call originating at station I00, it is found that the call to station 260 cannot be completed due to an all-trunks-busy or line-busy condition, the associated connecting switches transmit to station I00 a busy tone indication in the regular manner. When the receiver at station lllfl is hung up, the circuit releases as before described. The system functions in a similar manner in the caseof emu originating at station 200.

Although the switches illustrated herein'are spoken of as switches of the usual step-by-step type, it is to be understood that the invention is not limited'to systems using switches of this kind.

. Any suitable kind of switch may be used. Also the invention may be applied to trunks employed in systems where operators take part in the establishment of connections. a

What is claimed is:

' -l. In atelephone system a trunk circuit, means for seizing saidcircuit at either end, a guard cir cuit including one of the conductors of said trunk closed by said seizing means upon the seizure' of one end of said trunk, and a relay in said guard circuit at the other end of said trunk having a contact for placing a busy condition upon said trunk at said other end.

2. In a telephone system, a trunk circuit, automatic switches for seizing said trunk at either end, a guard circuit including one of the conductors of said trunk, means in one of said automatic switches for closing said guard circuit upon the seizure of one end of said trunk, a relay in said guard circuit at the distant end of the trunk operable in response to the closure of said guard circuit, and a circuit closed by the contacts of said relay for applying a busy potential to said distant end of the trunk circuit.

3. In a telephone system, a two-conductor trunk circuit having an outgoing end and an incoming end, a switch for seizing said trunk at the outgoing end thereof, a second switch for seizing said trunk at the incoming end thereof, a guard relay at said incoming end of the trunk, a circuit including one of the conductorsof said trunk closed by said first-mentioned switch upon the seizure of the outgoing end of said trunk for operating said guard relay, said relay having con tacts for applying a busy potential to the incoming end of said trunk to prevent its seizure by said second-mentioned switch, and circuit means local to'the incoming end of said trunk for maintaining said guard relay energized.

4. In a telephone system, a trunk circuit having an outgoing end and an incoming end, a switch for seizing the outgoing end of said trunk to ex tend a call over the trunk, an incoming switch at the incoming end of said trunk for further extending said call, switching meansat said incom- 7 ing end for seizing said trunk, a guard relay, a circuit closed by said first-mentioned switch in the act of seizing the outgoing end of said trunk, said circuit including one of the talking conductors of the trunk and also the winding of said guard relay, a pulse receiving relay for said incoming switch, said guard relay having contacts for extending the talking conductors to said pulse receiving relay anda contact for placing a busy condition on the incoming end of said trunk, and means for disconnecting said guard relay from the trunk and for holding it energized under the control of said pulse relay.

5. In a telephone system, a two-way trunk circuit, switches for seizing said trunk at either end, a guard circuit including one of the conductors of said trunk, relays in said guard circuit, one at either end thereof, and means in the switch seizing the trunk circuit at either end thereof for closing said guard circuit to operate the relay at the distant end to render the trunk busy at said distant end.

6. In a telephone system, a two-way trunk circuit, a first switch for seizing one end of said trunk, a second switch for seizing the other end of said trunk, relays, one at either end of said trunk having contacts for placing a busy potential on the test conductor of the trunk to prevent its 5 seizure bythe associatedswitch,and a guard circuit said switches, diiferential guard relays, one at each end of said trunk, a circuit closed by either of said switches in the act of seizing said trunk which extends from the seizing switch over the associated test terminal, over a talking conductor of the trunk to the distant end thereof, said circuit including the windings of both of said relays, a source for supplying current to said circuit in such a direction as to cause the operation only of the relay at the end distant from the seized end, and contacts on the operated relay for applying a busy potential to the test terminal of the trunk at the associated end.

HENRY HOVLAND. 

